"In my opinion, it's just a question of the sooner they realize it, the faster you get tax reform," said Daniel Clifton, Strategas head of policy research. He said the generic ballot was even on Election Day, and now gives Democrats an advantage, which suggests they could win about 30 seats in the House if the election were held now.

"You've just seen it move a lot toward the Democrats in the last couple of weeks," said Clifton. "We're still 15 months away. Could it swing back toward the Republicans? You bet it can."

The midterm election is also a referendum on the president — and President Donald Trump's approval rating fell to 39 percent in the latest Gallup poll. Bloomberg's poll last week showed that health care is the most important issue for voters, and they don't like the way Trump has handled it.

Clifton said about 90 percent of the swing in House races in midterm elections can be determined by the generic ballot question. He said in 2009, the data correctly pointed to an out-of-consensus view that the Democrats would lose badly in 2010.

"This was a big move no matter what number you use. This happens to every new president. This happened to Obama. It happened to Clinton. It happened to Reagan. It didn't happen to George W. Bush because of 9/11," Clifton said.

Clifton said while Republicans have a map advantage in both the House and Senate, midterm elections tend to have lower voter turnout and are presidential referendums.

"This is a sign the Republicans need to get their act together. They need to get policy passed," he said. "They've got some real issues, the Republicans."